1. Field of the Disclosure
The present invention relates generally to power converters, and more specifically, the invention relates to regulating the output of power converters.
2. Background
Many electrical devices such as cell phones, personal digital assistants (PDA's), laptops, etc. are powered by a source of relatively low-voltage DC power. Because power is generally delivered through a wall outlet as high-voltage AC power, a device, typically referred to as a power converter, is required to transform the high-voltage AC power to low-voltage DC power. The low-voltage DC power may be provided by the power converter directly to the device or it may be used to charge a rechargeable battery that, in turn, provides energy to the device, but which requires charging once stored energy is drained. Typically, the battery is charged with a battery charger that includes a power converter that meets constant current and constant voltage requirements required by the battery. Other electrical devices, such as DVD players, computer monitors, TVs and the like, also require a power converter for device operation. The power converter in these devices also has to provide output voltages and currents that meet the requirements of the device. In operation, a power converter may use a controller to regulate output power delivered to an electrical device, such as a battery, that may be generally referred to as a load. More specifically, the controller may be coupled to a sensor that provides feedback information of the output of the power converter in order to regulate power delivered to the load. The controller regulates power to the load by controlling a power switch to turn on and off in response to the feedback information from the sensor to transfer energy pulses to the output from a source of input power such as a power line.
The product of the power converter output voltage and current is termed the power converter output power. In most power converter applications, it is necessary to limit the worst case maximum output power that can be supplied to ensure the device being powered is protected from excessive power delivery. Improving the tolerance of the maximum output power that a power converter can deliver allows the electrical device being powered by the power converter to be optimized for safe operation under fault conditions, improves the electrical device reliability and reduces the overall cost of the electrical device.
One particular type of power converter that may be used is a flyback power converter. In a flyback power converter, an energy transfer element galvanically isolates the input side of the power converter from the output side. Galvanic isolation prevents DC current from flowing between the input side and the output side of the power converter.
A flyback power converter produces an output by switching a power switch to store energy in the energy transfer element during an on time of the power switch and deliver energy to a power converter output for at least a fraction of the time the power switch is off. In a non-isolated flyback converter, an energy transfer element is still required to store energy from the power converter input to be delivered to the power converter output, but no galvanic isolation is required to be provided by the energy transfer element.
In operation, a power converter may use a controller to regulate output power delivered to the load. More specifically the controller may limit a maximum output power of the power converter in response to feedback information derived by sensing output voltage and or output current at the output of the power converter. Sensing output current at the output of the power converter typically reduces power converter efficiency since a resistive element is typically introduced to provide a voltage signal proportional to the power converter output current. If the output current of the power converter is not sensed at the output of the power converter, the maximum power delivery limit is determined by the specification tolerances of certain components within the power converter.
Two components whose parameters influence the tolerance of the maximum output power of the power converter are the tolerance of the inductance of the energy transfer element and the tolerance of a protective current limit threshold for current flowing in the power switch while it is in an on state. The controller may sense the current flowing in the power switch while it is in an on state and may also set the maximum protective current limit threshold. In this case the tolerance of the controller maximum protective current limit threshold will influence the tolerance of the maximum power converter output power.